1. Field of the Invention
The disclosures herein generally relate to a control circuit and a control method thereof, a liquid droplet ejection head and an image forming apparatus.
2. Description of the Related Art
Conventionally, in the technical field of inkjet heads, an inkjet head is known which inkjet head has mounted a driver IC (control circuit) performing selection of electro-mechanical conversion elements for supplying a drive voltage wave form.
This driver IC includes, for example, analog switches connected to respective electro-mechanical conversion elements, selects a prescribed electro-mechanical conversion element by causing the analog switches to be ON or OFF, and causes liquid droplets to be ejected by applying the drive voltage wave form only to the selected electro-mechanical conversion element. In order to cause the analog switches to be ON or OFF, for example, a method can be used in which shift registers are built in the driver IC and analog switch selection data are set to the shift registers by externally inputting a data signal to the shift registers.
As a specific control method of the driver IC, a method is known in which the data signal input to the shift registers includes two bits and a print job is performed by four gradations: no liquid droplet ejection, a small droplet, a medium droplet and a large-size droplet (e.g., refer to Patent Document 1). Also, another method is known in which the data signal input to the shift registers includes three bits, a gradation control is available up to eight gradations and a print job is performed by five gradations: no liquid droplet ejection (without even a subtle drive), a small droplet, a medium droplet, a large droplet and a subtle drive (e.g., refer to Patent Document 2).
In the latter method (controlling up to eight gradations), although it is possible to prevent an ejection failure due to dried or viscosity-increased ink on the nozzle surface by performing the subtle drive, there are problems such as increased number of data signal lines for controlling five gradations, a more expensive controller and an increased cost for wiring parts. Therefore, although it may be possible to implement a function to control five gradations in expensive higher-performance models, it may be difficult to implement the function in inexpensive lower-performance models.
On the other hand, there is a requirement for using a common driver IC for both higher-performance models and lower-performance models because providing a dedicated driver IC for the higher-performance models and another dedicated driver IC for the lower-performance models leads to problems such as: the cost cannot be lowered because of decreased production numbers; and the cost will be increased because of increased numbers of management stock models. Therefore, there is a method in which a mode setting terminal is included in a driver IC and the number of gradations, such as four gradations or eight gradations, can be switched by switching H/L of a mode signal for the mode terminal.
In this case, two bits of the data signal are used for setting the analog switch selection data for the four gradations and three bits of the data signal are used for the eight gradations so that the data are read into two shift registers for the four gradations while the data are read into three shift registers for the eight gradations. Therefore, in the driver IC which is capable of switching between the four gradations and the eight gradations, when it is used for the four gradations, the third bit of the analog switch selection data signal is not provided and the third shift register is not used (functionally disabled).
For the third register which is not used for the four gradations, data is not set. As a result, the data is undefined (it can be H (1) or L (0)), or the data is not properly set by the two bits of the data signal alone. If a drive voltage is applied to an electro-mechanical conversion element when the data is not correctly set and the analog switch corresponding to the undefined data is caused to be ON, then a liquid droplet is unintentionally ejected.
In order to avoid the above problem, it is possible to design in such a way that before the drive voltage wave form is applied to the inkjet head after the power ON, inside the driver IC, the data of all L or all H is forcibly set to the third shift register which is not used for the four gradations. In other words, it is possible to design in such a way that clocks or latches are used to perform an initial operation for fixing the data of the third shift register to all L or all H.
However, there is a problem in that it is complicated to provide the initial operation. Although, in order to avoid such a problem, there is a method in which the same number of data signals and the same number of switching control signals are used when the lower number of gradations is selected as when the higher number of gradations is selected, there is a problem of increased cost.
[Patent Document 1] Japanese Patent Application Publication No. 2008-001084
[Patent Document 2] Japanese Patent Application Publication No. 2012-061664